1. Field of the Invention
The present invention relates to a light-emitting element array, a driving device for driving the light-emitting element array, and an image forming apparatus for forming images by using the light-emitting element array and driver device.
2. Description of the Related Art
In some conventional electrophotographic image forming apparatus, including some electrophotographic printers, an array of light-emitting elements selectively illuminates a charged photosensitive drum to form a latent image, which is developed by application of toner to form a toner image, and the toner image is transferred to and fused onto a sheet of paper.
The light-emitting elements and their driving circuits may be disposed on separate substrates, which are placed side by side and are electrically interconnected by bonding wires. In an electrophotographic printer using light-emitting diodes (LEDs) as light-emitting elements, the driving circuits switch the light-emitting elements on and off by feeding or not feeding current between the anode and cathode terminals of each LED. The driving scheme of an exemplary LED optical print head will be described below.
This LED optical print head is a typical head capable of printing on A4 paper with a resolution of 600 dots per inch. This requires a linear array of 4,992 LEDs, to print lines of 4,992 dots. These LEDs are disposed in twenty-six LED array chips, each including 192 LEDs. The cathodes of the odd-numbered LEDs are interconnected, the cathodes of the even-numbered LEDs are interconnected, and the anodes of mutually adjacent pairs of LEDs are interconnected, enabling the odd-numbered LEDs and the even-numbered LEDs to be driven alternately.
In FIG. 1, CHP1 and CHP2 are the first two LED array chips; the other LED array chips (CHP3 to CHP26) are not shown. Each LED array chip is driven by a separate driver integrated circuit (IC); the first two driver ICs (IC1 and IC2) are shown and the rest (IC3 to IC26) are omitted. The driver ICs have data terminals, which are connected in cascade to enable dot data to be passed from one driver IC to the next.
The LED array includes the LEDs 31 to 38 on the LED array chips and two power metal-oxide-semiconductor (MOS) transistors 41 and 42. The drain of power MOS transistor 41 is connected to the cathodes of the odd-numbered LEDs 31, 33, 35, 37; the drain of power MOS transistor 42 is connected to the even-numbered LEDs 32, 34, 36, 38. The source terminals of the power MOS transistors 41, 42 are grounded. The gate of power MOS transistor 41 is connected to a cathode driving (KDRV) terminal of driver IC1 and receives a signal denoted ODD; the gate of power MOS transistor 42 is connected to the KDRV terminal of driver IC2 and receives a signal denoted EVEN.
The driver ICs have data input terminals (DATAI3 to DATAI0) for receiving four-bit parallel print data signals (HD-DATA) in synchronization with a clock signal (HD-CLK). The first driver IC (IC1) receives these signals from a printing control unit (not visible); the other driver ICs receive the print data signals from the data output terminals (DATAO3 to DATAO0) of the preceding driver IC, and the other signals from the printing control unit. The four bits of print data received with each clock pulse pertain to the four odd-numbered LEDs or four even-numbered LEDs in a group of eight consecutive LEDs. The driver ICs have internal flip-flops (not visible) that form a shift register for holding bit data for 2,496 dots, and latch circuits (not shown) into which the data are loaded from the shift register in synchronization with a latch signal (HD-LOAD). The latched data are output in synchronization with a strobe signal (HD-STB-N) to drive the LEDs in the LED array chips with driving current regulated by a reference voltage VREF received from a reference voltage generating circuit (not shown). A synchronizing signal HD-SYNC-N determines whether the even-numbered or odd-numbered LEDs are driven. The driver ICs also have power supply (VDD) and ground (GND) terminals for receiving power.
The reason for driving the even-numbered or odd-numbered LEDs separately is to avoid the large flow of current that might occur if all the LEDs were to be driven simultaneously. The power MOS transistors 41, 42 in this conventional LED print head are required by the separate even-odd driving scheme. When the odd-numbered LEDs are driven, power MOS transistor 41 is switched on by the ODD signal to allow current to flow through the odd-numbered LEDs. When the even-numbered LEDs are driven, power MOS transistor 42 is switched on by the EVEN signal to allow current to flow through the even-numbered LEDs.
Even though at most only half of the LEDs are driven at once, the power MOS transistors 41, 42 must still be capable of switching considerable amounts of current. The power MOS transistors 41, 42 themselves are therefore necessarily large in size and take up considerable space in the LED print head. The power MOS transistor chips and the extra space needed for mounting them add to the cost of the materials used in the print head. The presence of these power MOS transistors is a major obstacle to reducing the size and cost of the print head.
In an electrophotographic printer proposed by the present inventor in U.S. Patent Application Publication No. 2007/0057259 (counterpart of Japanese Patent Application Publication No. 2007-81081), the need for these power MOS transistors is avoided by using light-emitting thyristors as light-emitting elements. The light-emitting thyristors are connected to a common current driving line and their gate terminals are driven individually, one by one, according to the print data. This driving scheme is, however, quite different from the conventional LED array driving scheme.